Telescoping folded monopole with capacitance at input



Sept. 24, 1968 BARRAR ET AL 3,403,405

TELESCOPING FOLDED MONOPOLE WITH CAPACITANCE AT INPUT Filed Jan. 8, 19652 Sheets-Sheet 1 INVENTOR5. [em/Mex) BAfl/EA f/LBMT E. 6/00/5 M DO QM MSept. 24, 1968 BARRAR ET AL 3,403,495

TZLESCOPING FOLDED MONOPOLE WITH CAPACITANCE AT INPUT Filed Jan 5, 19652 Sheets-Sheet 2 D RF Jar/ea:

w I L% l INVENTOR Z/Gl/ Q Qfl 51422192 United States Patent 3,403,405TELESCOPING FOLDED MONOPOLE WITH CAPACITANCE AT INPUT Richard Barrar,Beverly Hills, Calif., and Albert R. Giddis,

Lowell, Mass., assignors, by mesne assignments, to the United States ofAmerica as represented by the Secretary of the Navy Filed Jan. 8, 1965,Ser. No. 424,449 1 Claim. (Cl. 343745) This invention relates in generalto broadband antenna systems and their impedance matching and moreparticularly to adjustable monopole antennas.

In antennas of all types it is essential for efiicient and properoperation that the antenna be matched, with respect to impedance, to aload or to the source of energy over its operating frequency range.Clearly an antenna of proper physical proportions can be constructedwhereby it is tuned either for one particularly frequency or over alimited bandwidth. In the alternative the antenna may be composed of anumber of individually tuned elements all cooperating to achieve tunedconditions over an extended range. Numerous solutions in increasing thebandwidth characteristics of antennas have been suggested and someemployed quite successfully but this problem has not been overcome inthe case of a particular type of antenna, namely, the folded monopole.For efficient operation the monopole antenna must be impedance matchedto the RF. source and must be characterized by a sufficient radiationresistance. In view of the foregoing, it is an object of this inventionto provide to tunable broadband folded monopole antenna capable ofoperation over an extended range which may be simply matched to itsdriving source.

Another object is to provide a structurally simple, inexpensive, easilytunable, folded monopole antenna having means, also tunable, forextending the frequency range of the antenna below its physicallimitations.

Other objects nd advantages will appear from the following descriptionof an example of the invention, and the novel features will beparticularly pointed out in the appended claim.

In the accompanying drawings:

FIG. 1 is a schematic representation of a half-wave folded dipoleantenna;

FIG. 2 is a schematic representation of a standard quarter-wave foldedmonopole antenna; and FIGS. 3 and 4 are schematic representations offolded monopole antennas embodying the principles of this invention.

Extreme conditions of low input resistance are often encountered inantennas and may be due to short electrical length, or to mutual effectsof other nearby radiators, or both. In consequence, coupling networksmust be designed to transform the antenna resistance up to the levelwhere it provides the desired load impedance for the driving source. Thegeneral class of folded antennas, including folded dipoles and foldedmonopoles, represents the attempt to secure part of a desired impedancetransformation within the antenna itself.

Since half-wave folded dipoles are used extensively and the principle oftheir operation is well known, they will be first considered forpurposes of subsequent explanation. It consists of a half-wave dipolewith another half-wave dipole 11 proximate the first and joined to thefirst at the ends, by elements 12 and 13, as shown in FIG. 1. Energy issupplied to the driven dipole 11 from an RF. source 14 which isconnected thereto by transmission line 15. The voltage at the end ofeach dipole is maintained the same by the end connections 12 and 13.Consequently, the currents in each dipole will ,s CC

be in phase, maximum at the middle, and minimum at the ends.

-Let the maximum current in the section 11 driven by a voltage e be I,and that in the folded section be KI, where K is determined by therelative diameters of the two sections. Assuming P to be the power inputto the dipole, and letting R be the input resistance to the dipole,then, P=I R. Assuming further that all the input power is radiated, thenP: (J+KI) R,, where R is the radiation resistance of an ordinary, singlehalfwave dipole. Equating the input power to the radiated power, I R=(I+KI) R,,, then R= (1+K) R,,. This expression shows that by using thefolded structure, the input resistance R of the half-wave dipole isincreased by the factor of approximately (1+K) The folded monopole isessentially a folded dipole operated against ground and is illustratedschematically in FIG. 2. In this discussion, and only for this purpose,the ground plane is assumed infinite in extent and infinite inconductivity; in short, a perfect ground plane.

It should also be borne in mind that in the case of the quarter-wavefolded monopole, the impedance transformation is that for the half-wavefolded dipole with the input resistance and radiation resistance bothdivided by a factor of two (2). Here the two radiating quarter-wavesections, namely, the fed section 16 and unfed section 17 are joined byelement 18 at their respective upper ends. The opposite end of the unfedsection is spaced from the ground plane 19 and joined to it by conductor20 while source 21 is electrically disposed between the end of fedsection 16 and the ground plane.

The folded monopole antenna structure offers distinct advantages whenshort vertical radiators, having the inherent undesirable features oflow radiation resistance and large reactance must be used due tophysical limitations placed on the height of the antenna, as forexample, where it is disposed in a confined area.

The particular illustrated embodiment to be discussed below delineatesthe principles of this invention without restricting the invention tothe particular forms of structures or networks or numerical valuesintroduced by way of this sole example of an application utilizing theadjustable broadband impedance-matching antenna system.

In the embodiment illustrated in FIG. 3, a transmission line 22 passesthrough an aperture 23 in ground plane 24 and terminates level with theupper surface 25 of ground plane 24. The lower-most base element 26 ofseries telescoping fed antenna section 27 is electrically coupled to theend of transmission line 22 at point 28. The unfed section 29 isidentical to the above described section 27 with its lowermost baseelement 30 supported by and electrically connected to the upper surfaceof the ground plane. The antenna sections are spaced apart with anelectrically coupling section 31 joining their upper free ends. Bothantenna sections extend vertically outwardly of the ground plane andtheir physical dimensions height H above ground and spacing determinedfrom the presently available literature since folded monopole structuresare well known in the art.

In operation, the folded monopole is adjusted in length to resonate atits effective quarter-wave length. As a practical matter, it is notalways feasible to adjust this lengths to produce a purely resistiveantenna input impedance.

A typical impedance curve characterizing the variation with length andfrequency of the resistive and reactive components of the inputimpedance of a quarter-wave monopole fed at its base as the embodimentof FIG. 3 may be plotted. The values so obtained would reflect the orderof magnitude and basic frequency-length dependence of any foldedmonopole whose feed point is located at the level of the ground plane.The discussion to follow pertains to matching the antenna to a IOU-ohmtransmission line.

The complex impedance of the folded monopole for a length, for example,of H=0.18)\ where a is the freespace wavelength at which radio-frequencytransmission or reception occurs, is approximately 100j 50 ohms. Thus, astanding-wave ratio of less than 2 to 1 is obtained for a driving sourceimpedance of 100ij 0 ohms by changing the physical length of the foldedmonopole according to a corresponding change in frequency in order tomaintain a constant height-to-wavelength ratio. In a typical example,the maximum length limitation of the folded monopole may be restrictedto 35 feet. The matching technique just described, namely, adjusting thephysical length, will be effective for frequencies down to approximatelymegacycles per second (mc./s.). Any further decrease in frequencynecessitates an increase in length of antenna for any given diameter ofconductors and for the same conditions under which the matchingtechnique is applicable down to 5 mc./s. Since this increase in antennalength is prohibited by the condition of the example, namely, that themaximum length is restricted to 35 feet, which corresponds to about 5mc./s. for the folded monopole being used to exemplify the operation ofthe matching technique, then to secure a match with decreasingfrequency, it is observed that a change in length such that thlength-to-wavelength ratio is H/A =O.07 yields a complex impedance ofabout 100+j 300 ohms. Thus, by maintaining the length of the foldedmonopole at H-=0.07,\ the'antenna is matched below 5 mc./s. Arepresentative lower frequency, though not the absolute limit, is 2rnc./s. for high-frequency work. A representative upper frequency is 30mc./s. for highfrequency work.

The relatively large inductive reactance (300 ohms) at the input to theantenna, that is at its base, can be compensated by a capacitivereactance in series with the antenna impedance and Whose magnitude andsign are respectively equal and opposite to the inductive reactance. A186 micromicrofarad capacitor in series with the 100+;' 300 ohms overthe 2 to 5 mc./s. frequency band will result in a net impedance at theinput to the capacitive reactance which varies between 100+j 129 to100-j 129 ohms. If this impedance operates into a 100-ohm transmissionline, the standing-wave ratio is less than 4 to 1 over the 2 to 5 mc./s.frequency band.

In order to obtain an even lower standing wave ratio in the 2 to 5mc./s. frequency band, a 214 micromicrofarad capacitor can be used inthe 2 to 3.16 mc./s. band, and a 136 micromicrofarad capacitor can beused in the 3.16 to 5 mc./s. band. In this case, the antenna inputimpedance of 100+j 300 ohms is transformed into an impedance whichvaries between 100+ 69 and 100j 69 ohms over the 2 to 5 mc./s. band. Fora driving source impedance of 100:1 0 ohms, the standing-wave ratio isconsequently less than 2 to 1 over the entire frequency band ofoperation. This embodiment is clearly illustrated in FIG. 4 Where aselectable reactive element such as capacitors 32 is inserted in seriesbetween th transmission line 22 and the base element 26 of fed antennasection 27, Here a single pole-multithrow switch 33 provides a selectionfrom a plurality of capacitors such that a wide range of low frequenciesis available.

In practical application, these capacitors could consist of fixedcapacitors that'are switched in at the appro priate frequencies orcontinuously variable capacitors that change value corresponding tochanges in antenna impedance due to changing frequency when .the antennais set at its maximum length. The instant invention presents a uniquemethod for matching the output impedance of a driving source (such as,but not limited to, a transmitter output transformer or coaxialtransmission line) to a radiator of radio-frequency energy, namely, afolded monopole, by adjusting the length of the antenna and the heightabove a ground plane of the antenna, so that its input resistance isidentical to its quarter-wave resonant resistance, and so that its inputreactance is a relatively low value. To achieve a matched condition, thenon-zero reactance is minimized with a reactance of opposite sign and ofa magnitude equal to the antenna reactance. The complexity of thenetwork depends upon the degree of reactive mismatch to be tolerated,upon the bandwidth over which the radiating system is to be operated,upon the insertion loss of the network that is achievable and that is tobe tolerated, and upon such practical factors as size, shape and weight.

It will be understood that various changes in the details, materials andarrangements of parts (and steps), which have been herein described andillustrated in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claim.

We claim:

1. An adjustable broadband folded monopole antenna system for operationin conjunction with a ground plane which comprises:

a fed metallic antenna section having a plurality of extensibleintertelescoping elements,

an unfed metallic antenna section having a plurality of extensibleintertelescoping elements, said ground plane having an aperturetherethrough opening outwardly thereof, I said unfed section having oneend thereof afiixed to said ground plane and extending outwardlythereof, said fed section disposed with one end proximate said aperture,aligned with and isolated from said ground plane, I a series capacitanceconnected to said one end of said fed section and disposed entirelywithin said aperture, said capacitance having an impedance equal to theinductive impedance of said antenna at its lowest tuned frequency, and acoupling electrically connecting the free ends of said sections, wherebywhen a transmission line is connected to said one end to feed RF. energythereto and therefrom, the length of said antenna may be'adjustcd toprovide proper broadband matching and radia tion characteristics.

References Cited UNITED STATES PATENTS 3,117,279 1/1964 Ludvigson et al.343- 861 X FOREIGN PATENTS 873,233 3/1942 France. 886,770 7/1953Germany.

ELI LIEBERMAN, Primary Examiner.

1. AN ADJUSTABLE BROADBAND FOLDED MONOPOLE ANTENNA SYSTEM FOR OPERATIONIN CONJUNCTION WITH A GROUND PLANE WHICH COMPRISES: A FED METALLICANTENNA SECTION HAVING A PLURALITY OF EXTENSIBLE INTERTELESCOPINGELEMENTS, AN UNFED METALLIC ANTENNA SECTION HAVING A PLURALITY OFEXTENSIBLE INTERTELESCOPING ELEMENTS, SAID GROUND PLANE HAVING ANAPERTURE THERETHROUGH OPENING OUTWARDLY THEREOF, SAID UNFED SECTIONHAVING ONE END THEREOF AFFIXED TO SAID GROUND PLANE AND EXTENDINGOUTWARDLY THEREOF, SAID FED SECTION DISPOSED WITH ONE END PROXIMATE SAIDAPERTURE, ALIGNED WITH AND ISOLATED FROM SAID GROUND PLANE, A SERIESCAPACITANCE TO SAID ONE END OF SAID FED SECTION AND DISPOSED ENTIRELYWITHIN SAID APERTURE,